Some forms of optical fiber reflectors have previously been proposed. These were obtained by producing a quasi-periodical fluctuation in the refractive index of the core of the optical fiber. This fluctuation was achieved by exposing a photosensitive Ge-doped fiber core to a standing wave pattern resulting from the superposition of counter-propagating beams of light. Light from a high power Argon-ion laser is coupled into opposite ends of the fiber. The light propagates in opposite directions and produces a standing wave pattern in the region of the fiber where the two counter-propagating light beams have an almost equal difference in path length. The wave pattern extends approximately through a length DL. This length DL is related to the spectral bandwidth Df/f of the laser, and to the wavelength .lambda. of light in the fiber by the relationship: ##EQU1## By exposing the fiber to the light beams for a few moments in an environment where the fiber is held mechanically and thermally stable, the refractive index of that fiber in the region of interfering light beams is permanently modulated. It is the change in refractive index which produces a distributed-feedback reflection of a portion of an incident beam of light. In practice, it is believed that a slight amount of reflection of a light beam occurs at each fluctuation in refractive index, i.e., optical discontinuity. These give rise to a series of reflected beams delayed with respect to one another by multiples of the wavelength. These reflections add together, in phase, giving rise to a strong reflected beam.
This approach to making optical fiber reflectors does, however, face a number of problems. For example, if the fiber core is photosensitive, it too will be exposed by beams of light transmitted through it during normal use. This exposure will tend to erase the mudulations in refractive index over a period of time, to eventually destroying the usefulness of the reflector.
Moreover, the reflector is effective only at the same wavelength as that of the exposing beam. Hence, the longer the region of interference DL, the higher is the precision needed for matching the wavelengths of the exposing beam and transmitted beam. In the above example, an interference region DL of at least 1.0 meters long is required. Otherwise, an insufficient number of optical discontinuities result, i.e. fluctuations in refractive index, together with low overall reflectivity. From the equation (1) above, a precision in frequency Df/f of the order of 10.sup.-6 is required in that case.
Another difficulty that may be encountered is that of detuning of such highly frequency selective reflectors, due to mechanical or thermal perturbations. Detuning can arise from varying of the spatial period of the region of modulated index of refraction. For example, a reflector of one (1) meter in length can be detuned by a temperature difference as small as 0.05.degree. C., assuming a coefficient of thermal expansion of 10.sup.+5 per degree. Such sensitivity to environmental conditions is incompatible with many possible applications of optical fiber reflectors, for instance, in underwater sonar sensing.